The molecular mechanisms of chromosome replication and genetic recombination have not yet been elucidated. Although a variety of enzymes involved in DNA metabolism have been purified and characterized, these biological processes have not been duplicated in a cell-free system. This difficulty in part reflects a lack of information concerning the reaction mechanism of the purified enzymes as well as the necessity of identifying other enzymes and structural components. The detailed mechanism of action of enzymes such as the DNA polymerases and DNA ligases will be studied in order to design experiments to determine their role in vivo. For example, the structure and function of the ligase-adenylate intermediate in the joining reaction, and the replication of DNA at single-strand breaks by purified DNA polymerases, will be investigated. Emphasis will also be placed on the use of mutants defective in DNA metabolism and known DNA enzymes. Mutants of E. coli and phage T4 defective in DNA polymerase and ligase will be used to clarify the role of these enzymes in vivo, including the mechanism of suppression of T4 ligase mutations by rII mutations. Finally, the enzymes, modified DNA molecules, and mutants will be used to identify and characterize other enzymatic reactions responsible for the molecular events observed during chromosome replication and recombination.